Clay pipe segments, pitch-fibre pipes, and extruded plastics pipes have for many years been used in sub-soil drainage systems. In the systems making use of clay pipe segments, water-perviousness is provided by the porosity of the clay and by imperfect fitting of adjacent pipe segments. Although durable, clay pipes are expensive. In the pitch-fibre systems the pipes are drilled to render them water-pervious. In extruded plastics systems, the pipes are provided with transverse saw cuts to render them water-pervious. In both the pitch-fibre and extruded plastics systems, it is not possible readily to provide sufficiently narrow drill holes or saw cuts and this usually makes it necessary to provide a porous filler such as ash or gravel around the pipes during laying. This increases the labour cost. Furthermore, the drill holes and saw cuts are inevitably rough and of uniform width throughout the thickness of the pipe, leading to sand and other particles becoming lodged in the openings and clogging them up.
It is an object of the present invention to provide a relatively inexpensive sub-soil drainage pipe segment which will not suffer from the disadvantages set out above.